docs: update CI badge in README (#42)

Author: cwaldren-ld

README.md

@@ -1,13 +1,13 @@
 [![GoDoc](https://godoc.org/github.com/launchdarkly/eventsource?status.svg)](http://godoc.org/github.com/launchdarkly/eventsource)
-[![CircleCI](https://circleci.com/gh/launchdarkly/eventsource.svg?style=svg)](https://circleci.com/gh/launchdarkly/eventsource)
+[![Actions Status](https://github.com/launchdarkly/eventsource/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/launchdarkly/eventsource/actions/workflows/ci.yml)
 
 # Eventsource
 
 Eventsource implements a [Go](http://golang.org/) implementation of client and server to allow streaming data one-way over a HTTP connection using the Server-Sent Events API http://dev.w3.org/html5/eventsource/
 
 This is a fork of: https://github.com/donovanhide/eventsource
 
-# Supported Go versions
+## Supported Go versions
 
 The library supports the 'latest' and 'penultimate' Go versions defined in [this file](./.github/variables/go-versions.env).